1. Field of the Invention
The present invention relates to an integrator circuit having a DC offset circuit and to an active filter including such integrator circuits with a phase compensation by a resistor. More specifically, the present invention is directed to an RF active filter operable in a video frequency field.
2. Description of the Related Art
Very recently, active filters are constructed by employing differential amplifier circuits, and then fabricated in integrated circuits.
In particular, RF (radio frequency) active filters operable in high frequencies with having the frequency band of more than several MH.sub.z are expected in the fields; e.g., video signal circuits, High Definition Television circuits and CATV (cable television network) systems.
Various proposals have been made in integrator circuits constituting the conventional active filters. In Japanese KOKAI (Disclosure) patent application No. 58-161413 opened on Sept. 26, 1983, there is disclosed an integrator circuit as illustrated in FIG. 1.
In the conventional integrator circuit, after the signal V.sub.IN inputted to the input terminal 55 is amplified by the first differential amplifier circuit "A" which has been linearized by the resistors 56 and 57, the amplied input signal is supplied to the second differential amplifier circuit "B" constructed of the transistors Q.sub.25 and Q.sub.26, and then the signal which has been integrated by the capacitor 58 is output to the output terminal 59 (V.sub.OUT).
Considering various performances required for a typical integrator circuit for constituting an active filter, an ideal integrator circuit includes a first pole at a very low frequency, and neither other poles nor a zero point. However, since an actual integrator circuit contains a plurality of poles and also a zero point due to a certain limitation in performances of transistors employed therein, a satisfactory active filter can be constructed only under the severe conditions that these poles and zero point are set at such a higher frequency than a cutoff frequency of the active filter by more than 50 to 100 times.
Under the known technical backgrounds, when an active filter having a cutoff frequency of, for instance, 10 MHz, e.g., a lowpass filter, a highpass filter, or a bandpass filter is constructed, a second pole and also a zero point must be designed at a high frequency such as 500 MHz to 1 GHz. In other words, to manufacture the active filter having the satisfactory performances, a specific care must be taken to a very high frequency with respect to the operating frequency of this active filter.
In, on the other hand, the conventional integrator circuit shown in FIG. 1, considerably high resistance values are selected as the emitter resistors 56 and 57 of the transistors Q.sub.23 and Q.sub.24 of the first differential amplifier "A" in order to maintain the better linearity. That is to say, to linearize the non-linearity of the transistors Q.sub.23 and Q.sub.24 employed in the first differential amplifier circuit "A" by way of the resistors 56 and 57, the resistance values of these resistors 56 and 57 must be selected to be significantly higher than the equivalent resistance "1/gm (mutual conductance)" of these transistors Q.sub.23 and Q.sub.24.
Furthermore, it is obvious that since the temperature dependent characteristic of the transconductance Gm of the first differential amplifier circuit "A" constructed of the above-described transistors Q.sub.23, Q.sub.24 and resistors 56, 57 is influenced by both the first temperature dependent characteristic of the bipolar transistors Q.sub.23 and Q.sub.24, and also the second temperature dependent characteristic of the resistors, a simple temperature compensation circuit can hardly compensate for such temperature dependent characteristics. As a consequence, as a generic solution, the transconductance Gm is mainly influenced by the resistors, because the effect of the temperature dependency thereof is rather small. In a typical example, the resistance values of the resistors 56 and 57 are selected to be 2 to 4 killohms. Accordingly, the signal V.sub.IN inputted to the input terminal 55 is attenuated to, for instance, approximately 20 dB and then input to the base electrodes of the transistors Q.sub.25 and Q.sub.26.
As a consequence, the resultant transconductance Gm is extremely lowered in the overall integrator circuit even if the transistors having the high mutual conductance "gm" are employed. In addition, since a quality factor (Q) of an active filter having a higher quality factor is drastically changed at higher frequencies depending upon parameters of circuit elements thereof, a specific Q-controlling circuit is necessarily required in the conventional integrator circuit shown in FIG. 1.
Then, as previously described, since the temperature dependent characteristic of the transconductance "Gm" of the overall integrator circuit is determined by both the first and second temperature dependent characteristics of the bipolar transistors Q.sub.23 and Q.sub.24, and resistors 56 and 57, this temperature dependency cannot be correctly cancelled, or compensated.
As the base of the transistor Q.sub.24 is grounded in an AC sense in the first differential amplifier circuit "A", the input signal V.sub.IN supplied to the input terminal 55 is output via the emitter follower circuit arranged by one transistor Q.sub.23 and also the base-grounded circuit of the other transistor Q.sub.24. However, assuming the differential input and differential output, this differential amplifier circuit "A" may be considered as a half circuit in view of also the frequency characteristics. As illustrated in FIGS. 2 and 3, this differential amplifier circuit "A" has equivalently the characteristic of the single-staged emitter grounded circuit. In the circuit shown in FIG. 3, no resistor is included. However, as previously described one end of this circuit is grounded in an AC sense, the resultant equivalent circuit becomes complex, so that the above-described unnecessary poles as well as zero point are induced, which deteriorates the frequency characteristic. Furthermore, the input signal V.sub.IN passes through two stages of the first and second differential amplifier circuits "A" and "B", which also causes such a problem that the frequency characteristic is flowered.
In the differential amplifier circuit shown in FIG. 1, a PNP transistor Q.sub.27 is employed as the load resistor, i.e., an active load, the overall DC gain is calculated by multiplying the output resistor r.sub.01 of the transistor Q.sub.27 by the transconductance "Gm". However, the resistance value of the output resistor r.sub.01 is inherently determined by the manufacturing process of the entire integrator circuit. In other words, since this resistance value cannot be freely set, the resultant frequency characteristic is lowered.
Also, a monolithic active filter operable under the high frequency ranges of 100 kHz to 10 MHz of the IF filtering and video processing circuits of the TV circuit with utilizing the CMOS technology is disclosed in "High-Frequency CMOS Continuous-Time Filters" of IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. Sc-29 No. 6, December 1984 by HAIDEH KHORRAMABADI et al., from pages 939 to 948. In the described conventional active filter, the phase compensation is carried out to properly set the DC gain of the integrator by designing dimensions of the MOSFETS employed therein.
However, there are the following problems in the above-described conventional integrator circuits and active filters.
That is, in accordance with the conventional active filter employing the conventional integrator circuits shown in FIG. 1, since this active filter is arranged by the first differential amplifier circuit "A" which has been linearized by the resistors 56 and 57 having the relatively large resistance values, and also the second differential amplifier circuit "B" in which the PNP transistor Q.sub.27 is connected as the active load for this amplifier circuit, the resultant transconductance "Gm" of the entire integrator circuit is low and therefore this active filter cannot be satisfactorily operated in the high frequency range.
As previously described, the temperature dependent characteristic of the transconductance "Gm" of the entire integrator circuit is determined by both the first temperature dependent characteristic of the transistors Q.sub.23 and Q.sub.24 for constituting the first differential amplifier circuit "A" and the second temperature dependent characteristic of the resistors 56 and 57. As a result, it is very difficult that the temperature dependent characteristic of the transconductance "Gm" is correctly compensated.
Also it is known that a specific Q(quality factor)-control circuit means is required in the RF active filter having the high quality factor because this quality factor in significantly changed by the parameters of the circuit elements employed in the conventional integrator circuit shown in FIG. 1.
Then, as in the first differential amplifier circuit "A" at the input stage, the base electrode of the transistor Q.sub.24 is grounded, the equivalent circuit thereof becomes complex, and thus the unnecessary poles and zero point are produced. As a result, the frequency characteristic of the entire differential amplifier circuits is also deteriorated in view of this technical point.
In, on the other hand, the above-described CMO type monolithic active filter, there is another problem that the complex and high-cost phase compensation must be introduced.